This invention relates to determining the integrity of splices in optical fibers, and more particularly to method and apparatus for making field measurements of splice loss at the location of the splice.
In an optical communication system, it is most often necessary to splice the free end of a very short pigtail or input fiber, that is connected to a light source such as a laser diode, to the free end of a very long fiber (e.g. 10 kilometers) of the same size and type that is connected to optical receiver equipment at a remote location. The conventional method of determining splice loss is to measure the light output or reference power P1 at the free end of the input fiber prior to making a splice (this is the input power to the splice), and to measure the output power P2 at the remote end of the output fiber when this reference power on the input fiber is applied to the splice. The decibel value L1 of insertion loss of the optical system is L1=10 log P2/P1. The insertion loss L2 of the splice is L2=(10 log P2/P1)-L3, where L3 is the decibel loss in the output fiber. This technique has the disadvantage of not being suitable for practice at the location of the splice since it requires measurement of the radiant power at the far end of the output fiber. Additionally, this technique requires knowledge of the loss of the output fiber since it must be subtracted from the overall insertion loss of the optical system in order to determine the splice loss. This technique is also inaccurate since the loss of the output fiber (which may be from 20-50 dB for a 10 kilometer length of optical fiber) will normally be much greater than the splice loss (e.g., 0.2 dB). Another method of measuring splice loss is to connect an optical time domain reflectometer to the receiver end of the output fiber. The reflectometer transmits light pulses on the output fiber and measures reflections from the splice for producing an indication or measure of splice loss. This method requires the use of relatively expensive test equipment located remote from the splice location. It has been discovered that the decibel value of the ratio of radiant light power scattered from a splice to the radiant light power incident on the splice is related to the insertion loss of the splice. Scattered light is light that passes out of the fibers in the area of the splice. An object of this invention is the provision of method and apparatus for conveniently producing a measure of splice insertion loss, and which method may be practiced solely at the location of the splice.